the present invention relates in general to ignition devices for internal combustion engines.
The application of anti-pollution regulations which have become progressively more stringent has led to a need to use ever more sophisticated ignition systems in cars with petrol engines so as to ensure that the air-fuel mixture burns properly even under the most difficult conditions. This is because of problems of controllability and driving convenience, as well as to ensure that the limits set by the regulations are respected and, above all, to avoid a rapid decline in the conversion efficiency of the catalytic converter which is very sensitive to the presence of unburnt products.
Despite the precautions adopted, however, sporadic spark failure may occur because of occasional malfunctioning of the electronics, faulty contacts in the wiring or the soiling of the plugs. Although this is often not noticed by the average motorist, it is very important to be able to diagnose the phenomenon immediately and provide a timely indication of the need for maintenance of the system.
Various systems have been proposed in the past for detecting spark failure (that is, the fact that a spark has not been struck in a particular plug).
For example, in ignition systems with individual coils for each plug, it is possible to arrange for the terminal of the secondary winding opposite the respective spark plug to be connected to the ignition control unit and earthed through a resistor instead of being connected to the respective primary winding. The arc (or spark) current is therefore earthed through the resistor and its passage causes a pulsed voltage which accurately reproduces the arc current and can be used for diagnostic purposes to detect spark failure.
This solution has various disadvantages, amongst which may be mentioned:
an increase in radio-frequency interference; PA0 the need for more complex wiring with corresponding cost increments; PA0 the need to provide additional terminals on the connectors of the ignition control unit, and PA0 an intrinsic danger of the solution in that, if perfect connections are not ensured between the various terminals involved, electrical discharges may arise which, in the worst cases, may even happen within the ignition control unit with the possibility of serious damage thereto. PA0 the presence of fairly complex wiring (in the case of an engine with four cylinders, it is necessary to provide at least two return wires to the ignition control unit); PA0 the additional cost (although quite low) of the loop sensor which could even be incorporated in the resin in the conduit of the coil but this would give rise to insulation problems; PA0 a certain difficulty in the processing of the signals detected, and PA0 an increase in the interference emitted.
In an equivalent solution, the detection resistor may be transferred outside the control unit. In this case, however, a further need arises to provide a system for mounting the resistor, whilst all the other disadvantages listed above remain.
The solution just described cannot be applied to so-called lost-spark systems since (as is known) a terminal of the high-tension winding is no longer available as both the terminals are connected solely to the two high-tension terminals of the two plugs served.
Spark failure can be detected with a certain degree of effectiveness, however, even with these arrangements. Such systems provide for the use of an ignition coil with a dual high-tension output for each pair of plugs. In this case, the diagnosis can be made by the application of a capacitive loop sensor to one of the high-tension outputs to detect the high-tension pulses sent to the plugs. A solution of this type is described, for example, in European patent application No. EP-A-0 277 468, assigned to the same Assignee as the present application.
Although advantageous, this solution is not without some disadvantages. Amongst these may be cited, for example:
The present invention aims to resolve the problems mentioned above, preferably with the use of a circuit diagram of the type described in prior European patent application No. EP-A-0 383 730, assigned to the same Assignees as the present application.
The basic operating principles of this prior solution are shown in FIG. 1 of the appended drawings, which corresponds to FIG. 5 of the European application cited above.
In this drawing, the battery voltage, indicated VB, is used to charge the primary winding S1 of a coil B under the control of a Darlington transistor D with an associated Zener diode Dz for limiting the initial surge voltage. The coil B is constituted by a mutual impedance with a unitary or substantially unitary primary turns/secondary turns ratio.
The secondary winding S2 of the coil B is connected to the primary windings of respective voltage step-up transformers without air gaps mounted directly on the spark plugs. Only one of these voltage step-up transformers (indicated T1 and associated with a spark plug SP1) is shown in the diagram of FIG. 1, the numbers of turns in the primary winding and in the secondary winding being N1 and N2 respectively.
The energisation of the transformers associated with the plugs (T1 in this case) is controlled by respective electronic switches (for example, the triac TR1 shown in the diagram) piloted so as to ensure the correct firing sequence.
A resistor R is connected in series with the secondary winding S.sub.1 to limit the prepolarisation currents in the transformers associated with the plugs (T1) to a value of +B.sub.max. A diode, indicated D1, short-circuits the resistor R during the transfer or energy to the plugs. A capacitor, indicated C, is connected between the collector and the emitter of the Darlington transistor to limit the value of dV/dt in the switch TR1 at the instant at which the Darlington transistor is switched (off).
The excitation of the Darlington transistor D and of the triac TR1 is controlled, according to known criteria, by a control unit.
The coil B has the function of storing the electromagnetic excitation energy E=1/2LI.sup.2 in each cycle (a rotation of the engine through 180.degree.)
The energy is then discharged, the conductivity of the Darlington transistor D being blocked, and, after the respective electronic switch TR1 has been closed, the energy is transferred by the corresponding transformer T1 to the plug SP1 in which the discharge (spark) is to occur.
The sequence of closing (making conductive) the triac (TR1) associated with each plug (SP1) is effected in such a manner that the respective voltage step-up transformer (T1) is activated only for a brief period after the instant at which the Darlington transistor D starts to conduct so as to prevent (or at least to reduce) the production of spurious peaks in the plugs during the prepolarisation stage.
The distinctive characteristic of the circuit of FIG. 1 lies in the fact that, during the charging stage, the auxiliary coil B enables the transformer (T1) of each plug to be prepolarised to +B.sub.max and, hence, with a flow opposite that which is applied during the discharge.
FIGS. 2a and 2b (which correspond to FIGS. 6e and 6h of the European Application No. EP-A-0 383 730) show the waveform of the current i circulating between the secondary winding of the coil B and the primary winding of the transformer T1 (or of any one of the other transformers associated with the plugs) during the transfer of the spark energy. The graph of FIG. 2b, however, shows typical changes in the arc current i.sub.SP induced in the respective plug (e.g. SP1).
In order to explain the time graph of the current i (which naturally is repeated cyclically for each spark, starting from a theoretical time 0 preceding the time at which the spark is to be produced by a given interval--selected according to known criteria which need not be repeated herein) the following is true.
Interval 0-t1 (the Darlington transistor D is conductive which results in an increase in the intensity of the current in the primary winding S1 to a maximum value at the moment t1 at which the Darlington transistor starts to be cut off):
in practice, the current i corresponds to the sum of the prepolarising current of T1 and the current lost in the core;
interval t1-t2 (the generation, due to the interruption of the current in the primary winding S1, of a high pre-spark voltage in the secondary winding N2, until it reaches the dielectric breakdown value at the moment t2);
the sign of i is reversed as a result of the reversal of the voltage VP across the terminals of the primary winding of the voltage step-up transformer T1;
interval t2-t3 (discharge):
in practice, the current i corresponds to the sum of the arc current, which is given by the turns ratio relative to the primary winding of T1, the magnetisation current, and the current lost in the core; the peak which can be seen at the moment t2 is caused by the discharge of the capacitor C through the primary winding of the auxiliary coil B when the arc is struck;
moment t3 (annulment of the discharge current--quenching of the arc):
the current i corresponds to the sum of the magnetisation current and the lost current and decreases slowly to reach 0 at the moment when the next triac (associated with another plug) is switched on.